Nitrogen is an essential nutrient for the growth of living organisms, as it is a constituent of amino acids, proteins, and nucleic acids (DNA and RNA). It also contributes indirectly to chlorophyll formation through its role in the synthesis of enzymes and proteins required for photosynthesis. Although nitrogen gas (N₂) constitutes approximately 78% of the atmosphere, most plants cannot utilize it directly because of the strong triple bond connecting the two nitrogen atoms in the molecule. This highlights the importance of Biological Nitrogen Fixation (BNF), a process carried out by specialized microorganisms capable of converting atmospheric nitrogen into nitrogenous compounds that can be utilized in biological processes. This process plays an important role in maintaining soil fertility, supporting plant growth, and reducing dependence on manufactured nitrogen fertilizers, particularly in agricultural systems that rely on legumes. First: The Concept of Biological Nitrogen Fixation Biological nitrogen fixation is the conversion of atmospheric nitrogen gas (N₂) into ammonia (NH₃) by a specialized enzyme known as nitrogenase, which is found in certain groups of bacteria and archaea. The importance of this process lies in the fact that, despite its abundance in the atmosphere, nitrogen exists in a chemical form that higher plants cannot use directly. After fixation, it can be converted into ammonium ions (NH₄⁺) and organic nitrogen compounds that contribute to the formation of plant tissues. Some of this nitrogen can also enter the soil through the decomposition of roots, plant residues, and organic matter. It is important to distinguish between biological nitrogen fixation and plant nitrogen uptake. Nitrogen fixation converts N₂ gas into nitrogenous compounds, whereas uptake occurs when plant roots absorb available nitrogen from the soil, primarily in the form of nitrate (NO₃⁻) or ammonium (NH₄⁺). Second: Microorganisms Responsible for Nitrogen Fixation Several groups of microorganisms are capable of fixing nitrogen. They differ in their lifestyles and in their relationships with plants and the surrounding environment. 1. Root-Nodule Bacteria (Rhizobium) Rhizobium is one of the best-known groups of microorganisms involved in nitrogen fixation. It forms a symbiotic relationship with plants belonging to the legume family (Fabaceae), such as beans, peas, lentils, chickpeas, and certain types of clover. The relationship begins when the bacteria recognize chemical compounds secreted by plant roots, followed by an exchange of chemical signals between the two partners. These signals help initiate the infection process in a regulated manner and promote the formation of root nodules, specialized structures that develop on the roots and provide a suitable environment for nitrogen fixation. Inside the nodules, some bacterial cells differentiate into specialized forms called bacteroids, which carry out nitrogen fixation. The plant receives part of the fixed nitrogen, while supplying the bacteria with carbohydrates and energy sources produced through photosynthesis. 2. Azotobacter Bacteria The genus Azotobacter comprises free-living bacteria found in certain types of soil. These bacteria can fix nitrogen without forming a symbiotic association within root nodules. They require suitable sources of carbon and energy, and their activity is influenced by temperature, soil moisture, pH, and nutrient availability. Some species are characterized by their ability to protect the nitrogenase enzyme from the harmful effects of oxygen through various physiological mechanisms. However, the amount of nitrogen they fix in the soil varies according to environmental conditions, and the addition of these bacteria does not necessarily result in a substantial increase in crop yield under all circumstances. 3. Clostridium Bacteria The genus Clostridium includes species capable of fixing nitrogen under low-oxygen or anaerobic conditions. Some of these species occur in soil, sediments, and environments rich in organic matter, where they utilize organic compounds to obtain the energy required for their biological activities. This group demonstrates that nitrogen fixation is not limited to bacteria associated with plant roots; it can also be carried out by free-living microorganisms in various environments. 4. Cyanobacteria Cyanobacteria, also known as blue-green bacteria, are capable of photosynthesis, and some species can also fix atmospheric nitrogen. Examples include Anabaena and Nostoc. These organisms inhabit aquatic environments and moist soils, where they may contribute to nitrogen enrichment in certain ecosystems. Some species possess specialized cells called heterocysts, which provide low-oxygen conditions that help protect nitrogenase, an enzyme that is sensitive to oxygen. Al-Mustaqbal University – The First University in Iraq